1. Field of the Invention
The present invention relates to an optical relay amplifier primarily used for an optical communication network, and more particularly relates to a optical relay amplifier in which a semiconductor laser amplifier is used.
2. Description of the Conventional Art
In optical communication networks, optical signals are subjected to an optical relay amplification to compensate a loss of the intensity of the signals from attenuation and distribution of the signals. For example, in a bidirectional communication device in which star couplers are used, the patent of which was applied by the present applicant and disclosed in Japanese Unexamined Patent Publication No. 5-14285, an optical relay amplifier is provided between a plurality of star couplers to compensate the attenuation of the optical signals.
For example, Unexamined Japanese Patent Publication discloses that an optical relay amplifier is provided between a plurality of star couplers in order to compensate the attenuation of the optical signals in a bidirectional communication device using the star coupler.
FIG. 9 is a perspective view of the optical relay amplifier in which a semiconductor laser amplifier and a bypass optical waveguide path are combined, as disclosed in Unexamined Japanese Patent Publication No. 5-7055.
An optical signal input from the left of the drawing is divided by an optical coupler 35a. One of the divided optical signals passes through an optical waveguide path 36, semiconductor laser amplifier 34 and optical waveguide path 37, and then reaches an optical coupler 35b. The other of the divided optical signals passes through a bypass optical waveguide path 33, and reaches the optical coupler 35b. The optical signal that has passed through the semiconductor laser amplifier 34 is joined at the optical coupler 35b with the optical signal that has passed through the bypass optical waveguide path 33. Then the joined optical signal outputs from the optical coupler 35b to the right of the drawing. Since the semiconductor laser amplifier 34 is a bidirectional optical amplifier, an optical signal entering from the right of the drawing also passes through the same optical paths in the opposite direction and is amplified in the same manner. The bypass optical waveguide path 33 is provided for the purpose of fail-safe in the case of breakdown of the semiconductor laser amplifier 34. In this arrangement, cylindrical lenses 31a, 31 b are provided for connecting the semiconductor laser amplifier 34 with the optical waveguide paths 36 and 37.
The following problems are caused in the optical relay amplifier shown in FIG. 9. In order to arrange the semiconductor laser amplifier 34 on the optical waveguide path substrate 32, it is necessary to form a cutout portion 38 thereon. Accordingly, it is difficult to manufacture the optical waveguide path base board 32. It is necessary to optically polish an end surface of the optical waveguide path substrate so that the end surface can be optically connected with other units. In the case of the apparatus shown in FIG. 9, it is necessary to optically polish four end surfaces of the optical waveguide path substrate. Further, it is difficult to accomplish an optical alignment between the optical waveguide paths 36, 37 and the semiconductor laser amplifier 34.